Field effect transistor with channel region edge and center portions having different band structures for suppressed corner leakage

ABSTRACT

Disclosed are embodiments of field effect transistors (FETs) having suppressed sub-threshold corner leakage, as a function of channel material band-edge modulation. Specifically, the FET channel region is formed with different materials at the edges as compared to the center. Different materials with different band structures and specific locations of those materials are selected in order to effectively raise the threshold voltage (Vt) at the edges of the channel region relative to the Vt at the center of the channel region and, thereby to suppress of sub-threshold corner leakage. Also disclosed are design structures for such FETs and method embodiments for forming such FETs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending applicationfiled concurrently herewith by the same Applicants and assigned to thesame Assignee, namely, International Business Machines Corporation (IBMCorporation): “FIELD EFFECT TRANSISTOR WITH SUPPRESSED CORNER LEAKAGETHROUGH CHANNEL MATERIAL BAND-EDGE MODULATION, DESIGN STRUCTURE ANDMETHOD”, The complete disclosure of this related co-pending applicationis incorporated herein by reference.

BACKGROUND

1. Field of the Invention

The embodiments of the invention generally relate to field effecttransistors (FETs) and methods of forming these FETs with suppressedcorner leakage, as a function of channel material band-edge modulation.Also disclosed are design structures for such field effect transistors.

2. Description of the Related Art

As complementary metal oxide semiconductor (CMOS) devices are scaled insize, conventional gate structure structures are being replaced by metalgate structure structures. Specifically, a conventional gate structurestructure typically includes a thin silicon oxide (SiO₂) gate dielectriclayer and a doped-polysilicon gate conductor layer. Unfortunately, dopedpolysilicon gate conductor layers are subject to depletion effects.These depletion effects result in an increase in the effective gatedielectric layer thickness and, thereby limit device scaling. Thus,high-k-dielectric-layer, metal-gate-conductor-layer stacks withdifferent work functions for n-type field effect transistors (NFETs) andp-type field effect transistors, have been introduced. These stacks areimprovements over the conventional gate structures in that the highk-dielectric layer minimizes leakage current and the metal gateconductor layer is not subject to depletion effects. However, with evernarrower channel widths new concerns for future CMOS technologygenerations and, more particularly, for CMOS technology generations ator beyond the 65 nm node, driven by narrow-channel effects (NCE), areintroduced even with such high-k-dielectric-layer,metal-gate-conductor-layer stacks.

SUMMARY

In view of the foregoing, disclosed herein are embodiments of fieldeffect transistors (FETs) having suppressed sub-threshold cornerleakage, as a function of channel material band-edge modulation. Alsodisclosed are design structures for such FETs and method embodiments forforming such FETs.

One embodiment of a field effect transistor according to the presentinvention comprises a substrate. A semiconductor body is positioned onthe substrate and has a top surface and opposing sidewalls. Isolationregions, typically in the form of trench isolation, are positionedlaterally adjacent to the opposing sidewalls of the semiconductor body.The semiconductor body further comprises a channel region and a gatestructure adjacent to the channel region. This gate structure extendslaterally across the top surface of the semiconductor body (i.e., acrossthe channel width) and onto the trench isolation regions. In order tosuppress sub-threshold corner leakage at the interface between thechannel region and the trench isolation regions, the band structure ofthe channel material is different at the channel width edges as comparedto the central portion of the channel region. That is, the channelregion has edge portions adjacent to the trench isolation regions. Theseedge portions have a first band structure. The channel region furtherhas a central portion between the edge portions. This central portionhas a second band structure that is selectively different from the firstband structure.

Another embodiment of a field effect transistor according to the presentinvention comprises a substrate. A semiconductor body is positioned onthe substrate and has a top surface and opposing sidewalls. Trenchisolation regions are positioned laterally adjacent to the opposingsidewalls of the semiconductor body. As with the previously describedembodiment, the semiconductor body further comprises a channel regionand a gate structure adjacent to the channel region. In this embodiment,however, the trench isolation regions specifically comprise divotsadjacent to the upper sections of the opposing sidewalls of thesemiconductor body. Thus, in this embodiment, the gate structurecomprises a horizontal portion extending laterally across the topsurface of the semiconductor body and also a vertical portion extendinginto the divots adjacent to the upper sections of the opposingsidewalls. Again, in order to suppress sub-threshold corner leakage atthe interface between the channel region and the trench isolationregions, the band structure of the channel material is different at thechannel width edges as compared to the central portion of the channelregion. That is, the channel region has edge portions adjacent to thetrench isolation regions. Within the divots at the upper sections of theopposing sidewalls of the semiconductor body, these edge portions have afirst band structure. The channel region further has a central portionbetween the edge portions. This central portion has a second bandstructure that is selectively different from the first band structure.

Also disclosed herein are embodiments of design structure for theabove-mentioned field effect transistor embodiments. Each of thesedesign structures can be embodied in a machine readable medium used in adesign process, can reside on storage medium as a data format used forthe exchange of layout data of integrated circuits. Furthermore, each ofthese design structures can comprise a netlist and can include testdata, characterization data, verification data, and/or designspecifications.

Also disclosed herein are method embodiments for forming the abovedescribed FET embodiments. Specifically, one embodiment of the methodcomprises providing a substrate. Trench isolation regions are formed inthe substrate so as to define a semiconductor body with opposingsidewalls positioned laterally adjacent to the trench isolation regions.The area of the semiconductor body wherein a channel region will beformed is designated. This designated channel region has edge portionsadjacent to the trench isolation regions and a central portion betweenthe edge portions. Next, in order to ensure that sub-threshold cornerleakage is suppressed, the top surface of the semiconductor body ineither the central portion or the edge portions is altered such that theedge portions have a first band structure and the central portion has asecond band structure different from the first band structure. Then, agate structure is formed on the top surface of the semiconductor bodyadjacent to the designated channel region. Specifically, this gatestructure is formed so that it extends laterally across the top surfaceof the semiconductor body (i.e., across the channel width) and onto thetrench isolation regions.

Other embodiments of the method also comprises providing a substrate andthen forming trench isolation regions in the substrate so as to define asemiconductor body with opposing sidewalls positioned laterally adjacentto the trench isolation regions. However, in these embodiments, thetrench isolation regions are specifically formed with divots exposingthe upper sections of the opposing sidewalls of the semiconductor body.Again, the area of the semiconductor body wherein a channel region willbe formed is designated. This designated channel region has edgeportions adjacent to the trench isolation regions and a central portionbetween the edge portions. Next, in order to ensure that sub-thresholdcorner leakage is suppressed, the upper sections of the opposingsidewalls in the designated channel region of the semiconductor body canbe altered such that edge portions of the designated channel region havea first band structure and further such that a central portion of thedesignated channel region has a second band structure different from thefirst band structure. Alternatively, in order to ensure thatsub-threshold corner leakage is suppressed, the top surface of thesemiconductor body in the designated channel region can be altered suchthat edge portions of the designated channel region have a first bandstructure at the opposing sidewalls below the top surface and furthersuch that a central portion of the designated channel region has asecond band structure different from the first band structure.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments of the invention will be better understood from thefollowing detailed description with reference to the drawings, which arenot necessarily drawing to scale and in which:

FIG. 1 is a top view diagram illustrating the embodiments of the fieldeffect transistor of the present invention;

FIG. 2 is a cross section diagram illustrating one configuration for afirst embodiment of the field effect transistor of the presentinvention;

FIG. 3 is a cross section diagram illustrating another configuration forthe first embodiment of the field effect transistor of the presentinvention;

FIG. 4 is a cross section diagram illustrating one configuration for asecond embodiment of the field effect transistor of the presentinvention;

FIG. 5 is a cross section diagram illustrating another configuration forthe second embodiment of the field effect transistor of the presentinvention;

FIG. 6 is a flow diagram of a design process used in semiconductordesign, manufacture, and/or test;

FIG. 7 is a flow diagram illustrating a method of forming the fieldeffect transistors of FIGS. 2 and 3;

FIG. 8 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 9 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 10 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 11 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 12 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 13 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 7;

FIG. 14 is a flow diagram illustrating a method of forming the fieldeffect transistors of FIGS. 4 and 5;

FIG. 15 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 14;

FIG. 16 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 14;

FIG. 17 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 14; and

FIG. 18 is a cross section diagram illustrating a partially completedfield effect transistor formed according to the method of FIG. 14.

DETAILED DESCRIPTION

The embodiments of the invention and the various features andadvantageous details thereof are explained more fully with reference tothe non-limiting embodiments that are illustrated in the accompanyingdrawings and detailed in the following description.

As mentioned above, with scaling of complementary metal oxidesemiconductor (CMOS) devices, conventional gate structure structures arebeing replaced by metal gate structure structures. Specifically, aconventional gate structure structure typically includes a thin siliconoxide (SiO₂) gate dielectric layer and a doped-polysilicon gateconductor layer. Unfortunately, doped polysilicon gate conductor layersare subject to depletion effects. These depletion effects result in anincrease in the effective gate dielectric layer thickness and, therebylimit device scaling. Thus, high-k-dielectric-layer,metal-gate-conductor-layer stacks with different work functions forn-type field effect transistors (NFETs) and p-type field effecttransistors, have been introduced. These stacks are improvements overthe conventional gate structures in that the high-k-dielectric layerminimizes leakage current and the metal gate conductor layer is notsubject to depletion effects. However, with ever narrower channel widthsnew concerns for future CMOS technology generations and, moreparticularly, for CMOS technology generations at or beyond the 65 nmnode are introduced even with such high-k-dielectric-layer,metal-gate-conductor-layer stacks.

In view of the foregoing, disclosed herein are embodiments of fieldeffect transistors (FETs) having suppressed sub-threshold cornerleakage, as a function of channel material band-edge modulation.Specifically, the FET channel region is formed with different materialsat the edges as compared to the center. Different materials withdifferent band structures and specific locations of those materials areselected in order to effectively raise the threshold voltage (Vt) at theedges of the channel region relative to the Vt at the center of thechannel region and, thereby to suppress of sub-threshold corner leakage.Also disclosed are design structures for such FETs and methodembodiments for forming such FETs.

More particularly, referring to FIG. 1, each of the embodiments of thefield effect transistor of the present invention comprises a substrate101. The substrate 101 can, for example, comprise bulk singlecrystalline silicon wafer or silicon-on-insulator (SOI) wafer.

A semiconductor body 110 can be positioned on the substrate 101.Specifically, the semiconductor body 110 can, for example, be patternedfrom the top portion of a bulk silicon wafer (as illustrated) or from asingle crystalline silicon layer above a buried oxide layer. Thissemiconductor body 110 can comprise source/drain regions 160 and achannel region 150 between the source/drain regions 160. The channelregion 150 can have opposing sidewalls 152.

Trench isolation regions 120 can also be positioned on the substrate 101so as to define the limits of the semiconductor body 110. Specifically,these trench isolation regions 120 can be positioned laterallyimmediately adjacent to the semiconductor body 110 and, moreparticularly, immediately adjacent to the opposing sidewalls 152 of thesemiconductor body channel region 150. The trench isolation regions 120can, for example, comprise shallow trench isolation (STI) regions filledwith suitable isolation material(s) (e.g., SiO₂).

A gate structure 190 can be positioned adjacent to the channel region150. The different embodiments of the field effect transistor of thepresent invention, as well as the various configurations thereof, varywith respect to the gate structure 190 and further vary with respect tothe channel material in the center portion 171 of the channel region 150as compared to the channel material in the channel width edge portions172.

FIGS. 2 and 3 illustrate different configurations 200 a and 200 b of oneembodiment of the field effect transistor of the present invention. Ineach of these configurations 200 a and 200 b, the field effecttransistor comprises a substrate 101. A semiconductor body 110 ispositioned on the substrate 101 and has a top surface 155 and opposingsidewalls 152. Trench isolation regions 120 are positioned laterallyadjacent to the opposing sidewalls 152 of the semiconductor body 110.The semiconductor body 110 further comprises a channel region 150 and agate structure 190 adjacent to the channel region 150. This gatestructure extends laterally across the top surface 155 of thesemiconductor body 110 (i.e., across the channel width 180, see FIG. 1)and onto the trench isolation regions 120.

In order to suppress sub-threshold corner leakage at the interfacebetween the channel region 150 and the trench isolation regions 120(i.e., at the opposing sidewalls 152), the channel material and, therebythe band structure, is different in the channel width edge portions 172as compared to the channel material in the central portion 171 of thechannel region 150. That is, the channel region 150 has edge portions172 adjacent to the trench isolation regions 120. These edge portions172 have a first band structure. The channel region 150 further has acentral portion 171 between the edge portions 172. This central portion171 has a second band structure that is selectively different from thefirst band structure. The differences in band structures are a functionof different semiconductor materials being used in the differentportions 171, 172. The different materials with different bandstructures are selectively different so as to selectively adjust (i.e.,modulate, vary, etc.) the threshold voltage (Vt) of the channel widthedge portions 172 relative to the center portion 171 and, specifically,to suppress sub-threshold corner leakage at the channel width edgeportions 172.

For example, referring to the configuration 200 a of FIG. 2, the fieldeffect transistor 200 a can comprise a p-type filed effect transistor(PFET). Those skilled in the art will recognize that silicon germaniumis often used in the channel region of a PFET because it has a bandstructure that effectively reduces the negative threshold voltage (Vt)of the PFET (i.e. to a less negative value of Vt) to an optimal levelfor device performance. However, in the configuration 200 a, rather thanhaving the entire top surface 155 of the channel region 150 comprisesilicon germanium, only the top surface 155 in the center portion 171 ofthe channel region 150 comprises silicon germanium so that the centerportion 171 and edge portions 172 of the channel region 150 havedifferent band structures. More specifically, the top surface 155 of thesemiconductor body 110 in the edge portions 172 of the channel region150 adjacent to the gate structure 190 can comprise single crystallinesilicon 202 such that the edge portions 172 have a first band structureassociated with silicon. Only the top surface 155 in the central portion171 of the PFET channel region 150 comprises silicon germanium (e.g., anepitaxial silicon germanium layer 201) such that it has a second bandstructure that is different from the first band structure of the channelwidth edge portions 172. In such a PFET, the band structure of silicon202 raises the negative threshold voltage (to a more negative Vt) at thechannel width edge portions 172 relative to the negative Vt of thecentral portion 171, thereby suppressing sub-threshold corner leakage.Additionally, in the case of a PFET, the gate structure 190 can comprisea gate dielectric layer 191 (e.g., a high-k gate dielectric layer or anyother suitable gate dielectric material) adjacent to the top surface 155of the semiconductor body 110 and extending laterally onto the trenchisolation regions 120. The gate structure 190 can further comprise asuitable gate conductor layer 192 (e.g., a p-doped polysilicon gateconductor layer, a near valence band metal gate conductor layer or othersuitable gate conductor layer) on the gate dielectric layer 191.

Alternatively, referring to the configuration 200 b of FIG. 3, the fieldeffect transistor 200 b can comprise an n-type filed effect transistor(NFET). In the case of an NFET, the top surface 155 of the semiconductorbody 110 in the edge portions 172 of the channel region 150 adjacent tothe gate structure 190 can comprise a silicon carbide layer 302 suchthat the edge portions 172 have a first band structure associated withsilicon carbide. However, the top surface 155 of the semiconductor body110 in the central portion 171 of the channel region 150 adjacent to thegate structure 190 can comprise single crystalline silicon 301 such thatthe central portion 171 has a second band structure that is associatedwith silicon and, thereby, different from the first band structure. Insuch an NFET, the band structure of silicon carbide 302 raises thepositive threshold voltage (Vt) at the channel width edge portions 172relative the positive Vt of the central portion 171 and, therebysuppresses sub-threshold corner leakage. Additionally, in the case of anNFET, the gate structure 190 can comprise a gate dielectric layer 191(e.g., a high-k gate dielectric layer or any other suitable gatedielectric material) adjacent to the top surface 155 of thesemiconductor body 110 and extending laterally onto the trench isolationregions 120. The gate structure 190 can further comprise a suitable gateconductor layer 192 (e.g., an n-doped polysilicon gate conductor layer,a near conduction band metal gate conductor layer or other suitable gateconductor layer) on the gate dielectric layer 191.

FIGS. 4 and 5 illustrate different configurations 300 a and 300 b ofanother embodiment of the field effect transistor of the presentinvention. Each of these configurations 300 a-300 b comprises asubstrate 101. A semiconductor body 110 is positioned on the substrate101. The semiconductor body 110 has a top surface 155 and opposingsidewalls 152. Trench isolation regions 120 are positioned laterallyadjacent to the opposing sidewalls 152 of the semiconductor body 110. Aswith the previously described embodiment illustrated in FIGS. 2 and 3,the semiconductor body 110 further comprises a channel region 150 and agate structure 190 adjacent to the channel region 150.

In this embodiment, however, the trench isolation regions 120specifically comprise divots 425 adjacent to the upper sections 453 ofthe opposing sidewalls 152 of the semiconductor body 110. Thus, in thisembodiment, the gate structure 190 comprises a horizontal portion 495extending laterally across the top surface 155 of the semiconductor body110 and also a vertical portion 496 extending into the divots 425adjacent to the upper sections 453 of the opposing sidewalls 152. Aswith the previously described embodiment, in order to suppresssub-threshold corner leakage at the interface between the channel region150 and the trench isolation regions 120 (i.e., at the opposingsidewalls 152), the band structure of the channel material is differentat the channel width edges 172 as compared to the central portion 171 ofthe channel region 150. However, this embodiment takes advantage of thevertical portion 496 of the gate structure 190, when positioning thedifferent semiconductor materials with different band structures in thecenter 171 and edge 172 portions of the channel region 150.

For example, referring to the configuration 300 a of FIG. 4, the fieldeffect transistor 300 a can comprise a p-type filed effect transistor(PFET). As mentioned above, those skilled in the art will recognize thatsilicon germanium can be used in the channel region of a PFET because ithas a band structure that effectively reduces the negative thresholdvoltage (Vt) of the PFET to an optimal level for device performance.Thus, in the configuration 300 a, the entire top surface 155 of thechannel region 150 adjacent to the horizontal portion 495 of the gatestructure 190 comprises a silicon germanium layer 401. However, in orderto adjust the band structure of the channel width edge portions 172relative to the central portion 171, the upper sections 453 of theopposing sidewalls 152 of the semiconductor body 110 below the silicongermanium top surface 155 and adjacent to the vertical portion 496 ofthe gate structure 190 can comprise single crystalline silicon 402.Thus, the edge portions 172 have a first band structure associated withsilicon 402 and the central portion 171 has a second band structure thatis selectively different from the first band structure (i.e., a secondband structure associated with silicon germanium 401). In such a PFET,the band structure of silicon 402 raises the negative threshold voltage(Vt) at the channel width edge portions 172 relative to the negative Vtof the central portion 171, thereby suppressing sub-threshold cornerleakage. Additionally, in the case of a PFET, the gate structure 190 cancomprise a gate dielectric layer 191 (e.g., a high-k gate dielectriclayer or any other suitable gate dielectric material) adjacent to thetop surface 155 of the semiconductor body 110 (i.e., adjacent to thesilicon germanium layer 401) and extending vertically into the divots425 along the upper sections 453 of the opposing sidewalls 152 (i.e.,adjacent to the silicon 402). The gate structure 190 can furthercomprise a suitable gate conductor layer 192 (e.g., a p-dopedpolysilicon gate conductor layer, a near valence band metal gateconductor layer or other suitable gate conductor layer) on the gatedielectric layer 191.

Referring to the configuration 300 b of FIG. 5, the field effecttransistor 300 b can comprise an n-type field effect transistor (NFET).In the case of an NFET, the top surface 155 of the semiconductor body110 can comprise single crystalline silicon 501, whereas the uppersections 453 of the opposing sidewalls 152 can comprise a siliconcarbide layer 502. Thus, the channel width edge portions 172 have afirst band structure associated with silicon carbide 502 and the centralportion 171 of the channel region 150 has a second band structuredifferent from the first band structure (i.e., a second band structureassociated with silicon 501). In such an NFET, the band structure ofsilicon carbide 502 raises the positive threshold voltage (Vt) at thechannel width edge portions 172 relative to the positive Vt of thecentral portion 171 and, thereby suppresses sub-threshold cornerleakage. Additionally, in the case of an NFET, the gate structure 190can comprise a gate dielectric layer 191 (e.g., a high-k gate dielectriclayer or any other suitable gate dielectric material) adjacent to thetop surface 155 of the semiconductor body 110 (i.e., adjacent to thesilicon 501) and extending vertically into the divots 425 along theupper sections 453 of the opposing sidewalls 152 (i.e., adjacent to thesilicon carbide layer 502). The gate structure 190 can further comprisea suitable gate conductor layer 192 (e.g., an n-doped polysilicon gateconductor layer, a near conduction band metal gate conductor layer orother suitable gate conductor layer) on the gate dielectric layer 191.

Also disclosed herein are embodiments of design structure for theabove-mentioned field effect transistor embodiments and variousconfigurations thereof as illustrated in FIGS. 1-5. Each of these designstructures can be embodied in a machine readable medium used in a designprocess, can reside on storage medium as a data format used for theexchange of layout data of integrated circuits. Furthermore, each ofthese design structures can comprise a netlist and can include testdata, characterization data, verification data, and/or designspecifications.

Specifically, FIG. 6 shows a block diagram of an exemplary design flow600 used for example, in semiconductor design, manufacturing, and/ortest. Design flow 600 may vary depending on the type of IC beingdesigned. For example, a design flow 600 for building an applicationspecific IC (ASIC) may differ from a design flow 600 for designing astandard component. Design structure 620 is preferably an input to adesign process 610 and may come from an IP provider, a core developer,or other design company or may be generated by the operator of thedesign flow, or from other sources. Design structure 620 comprises anembodiment of the invention as shown in FIGS. 1-5 in the form ofschematics or HDL, a hardware-description language (e.g., Verilog, VHDL,C, etc.). Design structure 620 may be contained on one or more machinereadable medium. For example, design structure 620 may be a text file ora graphical representation of an embodiment of the invention as shown inFIGS. 1-5.

Design process 610 preferably synthesizes (or translates) an embodimentof the invention as shown in FIGS. 1-5 into a netlist 680, where netlist680 is, for example, a list of wires, transistors, logic gates, controlcircuits, I/O, models, etc. that describes the connections to otherelements and circuits in an integrated circuit design and recorded on atleast one of machine readable medium. This may be an iterative processin which netlist 680 is resynthesized one or more times depending ondesign specifications and parameters for the circuit.

Design process 610 may include using a variety of inputs; for example,inputs from library elements 630 which may house a set of commonly usedelements, circuits, and devices, including models, layouts, and symbolicrepresentations, for a given manufacturing technology (e.g., differenttechnology nodes, 32 nm, 45 nm, 90 nm, etc.), design specifications 640,characterization data 650, verification data 660, design rules 670, andtest data files 685 (which may include test patterns and other testinginformation).

Design process 610 may further include, for example, standard circuitdesign processes such as timing analysis, verification, design rulechecking, place and route operations, etc. One of ordinary skill in theart of integrated circuit design can appreciate the extent of possibleelectronic design automation tools and applications used in designprocess 610 without deviating from the scope and spirit of theinvention. The design structure of the invention is not limited to anyspecific design flow.

Design process 610 preferably translates an embodiment of the inventionas shown in FIGS. 1-5, along with any additional integrated circuitdesign or data (if applicable), into a second design structure 690.Design structure 690 resides on a storage medium in a data format usedfor the exchange of layout data of integrated circuits (e.g. informationstored in a GDSII (GDS2), GL1, OASIS, or any other suitable format forstoring such design structures). Design structure 690 may compriseinformation such as, for example, test data files, design content files,manufacturing data, layout parameters, wires, levels of metal, vias,shapes, data for routing through the manufacturing line, and any otherdata required by a semiconductor manufacturer to produce an embodimentof the invention as shown in FIGS. 1-5. Design structure 690 may thenproceed to a stage 695 where, for example, design structure 690:proceeds to tape-out, is released to manufacturing, is released to amask house, is sent to another design house, is sent back to thecustomer, etc.

Also disclosed herein are method embodiments for forming the abovedescribed FET embodiments. Specifically, referring to the flow diagramof FIG. 7, one embodiment of the method comprises providing a substrate101, such as a bulk single crystalline silicon wafer or asilicon-on-insulator (SOI) wafer (702). Then, trench isolation regions120 are formed in the substrate 101 (e.g., using conventional STIprocessing techniques) so as to define a semiconductor body 110 withopposing sidewalls 152 positioned laterally adjacent to the trenchisolation regions 120 (704, see FIGS. 8 and 9 in combination). The areaof the semiconductor body 110 wherein a channel region 150 will beformed is designated. This designated channel region 150 has edgeportions 172 adjacent to the trench isolation regions 120 and a centralportion 171 between the edge portions 172.

Next, in order to ensure that sub-threshold corner leakage issuppressed, the top surface 155 of the semiconductor body 110 in eitherthe central portion 171 or the edge portions 172 is altered such thatthe edge portions 172 have a first band structure and the centralportion has a second band structure different from the first bandstructure (706). Specifically, this altering is performed so that thefirst band structure and the second band structure are selectivelydifferent in order to selectively adjust a threshold voltage (Vt) of theedge portions 172 of the channel region 150 relative to the Vt of thecenter portion 171 of the channel region 150 in order to suppresssub-threshold corner leakage at the edge portions 172.

For example, for a p-type field effect transistor, a mask 1001 can beformed on the top surface 155 of the semiconductor body 110 above theedge portions 172 so as to expose the central portion 171 (707, see FIG.10). Formation of the mask 1001 can be accomplished, for example, byoxidizing the top surface 155 of the semiconductor body 110. Next, aresist layer can be deposited and patterned over the edge portions 172,exposing the oxidized top surface in the center portion 171. Then, a wetetch process can be performed in order to remove the oxide from theexposed central portion 171. Once the wet etch process is performed, thepatterned resist layer can be removed, leaving the remaining oxidizedsurface on the edge portions 172 of the semiconductor body 110 tofunction as the mask 1001. After the mask 1001 is formed, a silicongermanium layer 201 can be formed (e.g., using conventional epitaxialdeposition processes) on the exposed silicon in the central portion 171of the channel region 150 of the semiconductor body 110 and the mask1001 can be removed (709-711, see FIG. 11).

Alternatively, for an n-type field effect transistor, a mask 1002 can beformed on the on the top surface 155 of the semiconductor body 110 abovethe central portion 172 so as to expose the edge portions 171 (713, seeFIG. 12). Next, a silicon carbide layer 302 can be formed on the exposededge portions 172 of the semiconductor body 110 (e.g., by performing ashallow carbon implant process) (715, see FIG. 13). Once the siliconcarbide layer 302 is formed, the mask 1002 can be removed and thermalannealing performed to incorporate the carbon into the silicon crystallattice (717, see FIG. 13).

Following the altering process 706, a gate structure 190 can be formedadjacent to the designated channel region 150 and extending laterallyonto the trench isolation regions 120 (718-721, see FIGS. 2 and 3). Thisgate structure 190 can be formed by depositing a thin gate dielectriclayer 191 (e.g., a high-k gate dielectric layer or any other suitablegate dielectric material) adjacent to the top surface 155 of thesemiconductor body 110 and trench isolation regions 120. Following gatedielectric layer 191 formation, a gate conductor layer 192 can be formedon the gate dielectric layer 191 to create a gate stack. In the case ofa PFET, a p-doped polysilicon gate conductor layer, a near valence bandmetal gate conductor layer or any other suitable gate conductor layercan be used. In the case of an NFET, an n-doped polysilicon gateconductor layer, a near conduction band metal gate conductor layer orany other suitable gate conductor layer can be used. The resulting stack191-192 can be patterned and etched to form a gate structure 190 on thetop surface 155 of the designated channel region 150 and extendinglaterally above at least a portion of the trench isolation regions 120.

After completion of the gate structure 190, additional processing isperformed in order to complete the FET structures (722). This additionalprocessing includes, but is not limited to, halo implantation,source/drain extension implantation, gate sidewall spacer formation,source/drain implantation, silicide formation, interlayer dielectricdeposition, contact formation, etc.

Referring to the flow diagram of FIG. 14, other embodiments of themethod also comprise providing a substrate 101, such as a bulk singlecrystalline silicon wafer or silicon-on-insulator (SOI) wafer (1402).Then, trench isolation regions 120 are formed in the substrate 101(e.g., using conventional STI processing techniques) so as to define asemiconductor body 110 with opposing sidewalls 152 positioned laterallyadjacent to the trench isolation regions 120 (1404). However, in theseembodiments, the trench isolation regions 120 are specifically formedwith divots 425 that expose the upper sections 453 of the opposingsidewalls 152 of the semiconductor body 110 (see FIG. 15). Techniquesthat allow for such divot 425 formation in STIs 120 are well-known inthe art. For example, divots can be formed during shallow trenchisolation formation by providing a SiO₂ film on the silicon wafer, witha Si₃N₄ film above, patterning and etching trenches through both filmsand at least part way into the silicon, filling the trenches with SiO₂by CVD deposition and CMP/etch-back to the top of the remaining Si₃N₄film. The remaining Si₃N₄ films are selectively removed, and some of theexposed SiO₂ trench fill is etched back isotropically. This isotropicetching will result in a divot adjacent to the silicon islands. Again,the area of the semiconductor body 110 wherein a channel region 150 willbe formed is designated. This designated channel region 150 has edgeportions 172 adjacent to the trench isolation regions 120 and a centralportion 171 between the edge portions 172.

Next, in order to ensure that sub-threshold corner leakage issuppressed, the top surface 155 of the semiconductor body 110 in thedesignated channel region 150 can be altered such that edge portions 172of the designated channel region 150 have a first band structure at theopposing sidewalls 152 below the top surface 155 and further such that acentral portion 171 of the designated channel region 150 has a secondband structure different from the first band structure (1406-1407).

For example, in the case of a PFET, the divots 425 can be filled with aprotective layer 1005 (E.g., boron nitride) in order to protect theupper sections 453 of the opposing sidewalls 152. Then, a polishingprocess (e.g., chemical mechanical polishing (CMP)) can be performed soas to expose the semiconductor body 110 (see FIG. 16). Once thesemiconductor body 110 is exposed, a silicon germanium layer 401 can beformed (e.g., by a conventional epitaxial deposition process) on the topsurface 115 of the semiconductor body 110 and the protective layer 1005can be removed from the divots 425 (e.g., by performing a selective etchprocess) (see FIG. 17). Thus, the edge portions 172 of the designatedchannel region 150 have a first band structure associated with silicon402 at the opposing sidewalls 152 below the silicon germanium 401 on thetop surface 155 and the central portion 171 of the designated channelregion 150 has a second band structure associated with silicon germanium402 (i.e., the first band structure in the edge portions 172 and thesecond band structure in the center portion 171 are different).

Alternatively, in order to ensure that sub-threshold corner leakage issuppressed, the upper sections 453 of the opposing sidewalls 152 in thedesignated channel region 150 of the semiconductor body 110 can bealtered such that edge portions 172 of the designated channel region 150have a first band structure and further such that the central portion171 of the designated channel region 150 has a second band structuredifferent from the first band structure (1408-1409).

For example, in the case of an NFET, after the trench isolation regions120 are formed with divots 425 exposing the upper sections 453 of theopposing sidewalls 152 of the semiconductor body 110, a silicon carbidelayer 502 can be formed within the divots 425 on the upper sections 453of the opposing sidewalls 152. Thus, the edge portions 172 of thedesignated channel region 150 have a first band structure associatedwith the silicon carbide 502 on the upper sections 453 of the opposingsidewalls 152 and the central portion 171 of the designated channelregion 150 has a second band structure associated with silicon 501 atthe top surface 155 (i.e., the first band structure in the edge portions172 and the second band structure in the center portion 171 aredifferent).

Consequently, be it the top surface 155 of the semiconductor body 110 orthe upper sections 453 of the opposing sidewalls 152 of thesemiconductor body 110 being altered, the altering process is performedsuch that the first band structure of the edge portions 172 and thesecond band structure of the central portion 171 are selectivelydifferent in order to selectively adjust a threshold voltage (Vt) of theedge portions 172 of the channel region 150 relative to the Vt of thecentral portion 171 of the channel region 150 and, thereby to suppresssub-threshold corner leakage at the edge portions 172.

Following the altering processes 1406 or 1408, a gate structure 190 canbe formed adjacent to the designated channel region 150 (1410).Specifically, a gate structure 190 can be formed with a horizontalportion 495 above the top surface 155 of the designated channel regionand a vertical portion 496 extending into the divots 425 along the uppersections 453 of the opposing sidewalls 152 (1410-1412, see FIGS. 4 and5). This gate structure 190 can be formed by depositing a thin conformalgate dielectric layer 191 (e.g., a high-k gate dielectric layer or anyother suitable gate dielectric material) above the top surface 155 ofthe semiconductor body 110 and within the divots 425 adjacent to theupper sections 453 of the opposing sidewalls 152. Following gatedielectric layer 191 formation, a gate conductor layer 192 can be formedon the gate dielectric layer 191 to create a gate stack. In the case ofa PFET, a p-doped polysilicon gate conductor layer, a near valence bandmetal gate conductor layer or any other suitable gate conductor layercan be used. In the case of an NFET, an n-doped polysilicon gateconductor layer, a near conduction band metal gate conductor layer orany other suitable gate conductor layer can be used. The resulting stack191-192 can be patterned and etched to form a gate structure 190 abovethe top surface 155 of the designated channel region 150 and extendingvertically into the divots 425 adjacent to the upper sections 453 of theopposing sidewalls 152.

After completion of the gate structure 190, additional processing isperformed in order to complete the FET structures (1414). Thisadditional processing includes, but is not limited to, haloimplantation, source/drain extension implantation, gate sidewall spacerformation, source/drain implantation, silicide formation, interlayerdielectric deposition, contact formation, etc.

It should be understood that for each of the transistor, designstructure and method embodiments, described in detail above, high-kdielectric materials comprise dielectric materials having a dielectricconstant “k” above 3.9 (i.e., above the dielectric constant of SiO₂).Exemplary high-k dielectric materials that can be incorporated into thedisclosed embodiments include, but are not limited to, hafnium-basedmaterials (e.g., HfO₂, HfSiO, HfSiON, or HfAlO) or some other suitablehigh-k dielectric material (e.g., Al₂O₃, TaO₅, ZrO₅, etc.). Exemplarynear valence band metal gate conductor materials that can beincorporated into the disclosed PFET embodiments can include, but arenot limited to, rhenium, rhenium oxide, platinum, ruthenium, rutheniumoxide, nickel, palladium, iridium, etc. or suitable alloys thereof.Finally, exemplary near conduction band metals that can be incorporatedinto the disclosed NFET embodiments can include, but are not limited to,titanium nitride, titanium silicon nitride, tantalum nitride, tantalumsilicon nitride, aluminum, silver, hafnium, etc.

It should be understood that the corresponding structures, materials,acts, and equivalents of all means or step plus function elements in theclaims below are intended to include any structure, material, or act forperforming the function in combination with other claimed elements asspecifically claimed. Additionally, it should be understood that theabove-description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiments were chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated. Well-known components and processingtechniques are omitted in the above-description so as to notunnecessarily obscure the embodiments of the invention.

Finally, it should also be understood that the terminology used in theabove-description is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention.For example, as used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. Furthermore, as used herein, the terms “comprises”,“comprising,” and/or “incorporating” when used in this specification,specify the presence of stated features, integers, steps, operations,elements, and/or components, but do not preclude the presence oraddition of one or more other features, integers, steps, operations,elements, components, and/or groups thereof.

Therefore, disclosed above are embodiments of field effect transistors(FETs) having suppressed sub-threshold corner leakage, as a function ofchannel material band-edge modulation. Specifically, the FET channelregion is formed with different materials at the edges as compared tothe center. Different materials with different band structures andspecific locations of those materials are selected in order toeffectively raise the threshold voltage (Vt) at the edges of the channelregion relative to the Vt at the center of the channel region and,thereby to suppress of sub-threshold corner leakage. Also disclosed aredesign structures for such FETs and method embodiments for forming suchFETs.

Benefits which flow from the above invention include circuits andproducts with reduced power consumption, both in standby mode and activemode, circuits with greater tolerance to low power-supply voltage, andhigher speed circuits due to the ability to lower threshold voltagewhile maintaining low leakage currents. Furthermore increased densityand thus decreased manufacturing costs can follow by using physicallynarrower (smaller) transistors while achieving a given level ofperformance compared to the prior art.

What is claimed is:
 1. A field effect transistor comprising: asubstrate; a semiconductor body on said substrate and having a topsurface and opposing sidewalls; isolation regions positioned laterallyadjacent to said opposing sidewalls, said isolation regions comprisingdivots adjacent to upper sections of said opposing sidewalls; and a gatestructure adjacent to a channel region of said semiconductor body, saidgate structure comprising a horizontal portion extending laterallyacross said top surface of said semiconductor body and a verticalportion extending into said divots adjacent to said upper sections ofsaid opposing sidewalls, said channel region comprising edge portionsadjacent to said isolation regions and a central portion between saidedge portions, said edge portions comprising a first band structure, andsaid central portion comprising a second band structure different fromsaid first band structure.
 2. The field effect transistor of claim 1,said first band structure and said second band structure beingselectively different so as to selectively adjust a threshold voltage ofsaid field effect transistor.
 3. The field effect transistor of claim 1,said first band structure and said second band structure beingselectively different so that said first band structure suppressessub-threshold corner leakage at said edge portions.
 4. The field effecttransistor of claim 1, said field effect transistor comprising a p-typefield effect transistor and said gate structure comprising a high-k gatedielectric layer and a near valence band metal gate conductor layer onsaid high-k gate dielectric layer.
 5. The field effect transistor ofclaim 1, said field effect transistor comprising an n-type field effecttransistor and said gate structure comprising a high-k gate dielectriclayer and a near conduction band metal gate conductor layer on saidhigh-k gate dielectric layer.
 6. The field effect transistor of claim 5,said top surface of said semiconductor body comprising silicon and saidupper sections of said opposing sidewalls comprising a silicon carbidelayer such that said edge portions comprise said first band structureand said central portion comprises said second band structure differentfrom said first band structure.